Tables Used

orderdetails

Column	Type
orderdetailid	bigint
orderid	bigint
productid	bigint
quantity	bigint
unitprice	double precision
currencyid	bigint

orders

Column	Type
orderid	bigint
customerid	bigint
orderdate	timestamp without time zone
totalamount	double precision
status	text

Expected Output

Column
productid
orderdate
quantity
previousquantity
quantitychange

How Much More Did They Order? :Analyze Changes in Product Order Quantities Across Time Intermediate

Q: Do I need advanced SQL to attempt this?

This problem is labeled as Intermediate. It assumes you’ve written a few window function queries before, but you don’t need to be an expert. Use the hints and explanations if you get stuck — they’re there to help you think through the logic.

Track changes in quantity across each product's order timeline.

Xorthax wants to analyze purchasing patterns. For each Product ID, calculate the change in order quantity compared to the previous order (as determined by order date).

Write Your Query

Answer:

          select od.productid,
       o.orderdate,
       od.quantity,
       lag(od.quantity, 1) over (partition by od.productid
                                 order by o.orderdate) as previousquantity,
                                od.quantity - lag(od.quantity, 1) over (partition by od.productid
                                                                        order by o.orderdate) as quantitychange
from orderdetails od
join orders o on od.orderid = o.orderid;

This mirrors monitoring sales fluctuations by tracking how product demand shifts from one order to the next.

You will practice comparing sequential records to identify changes and trends across ordered data.

This problem is labeled as Intermediate. It assumes you’ve written a few window function queries before, but you don’t need to be an expert. Use the hints and explanations if you get stuck — they’re there to help you think through the logic.

Yes. Every problem comes with optional hints you can reveal one at a time, plus a fully worked step-by-step solution. You decide how much help you want while practicing.

All problems on PracticeWindowFunctions.com are completely free and can be solved without creating an account. Right now there are over 80 practice problems, with new ones added regularly.

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Tables Used

Expected Output

How Much More Did They Order? :Analyze Changes in Product Order Quantities Across Time Intermediate

Track changes in quantity across each product's order timeline.

What real-world scenario does this represent?

What SQL concepts will I practice here?

Do I need advanced SQL to attempt this?

Will I get hints and step-by-step explanations if I get stuck?

Are the problems free to access, and how many are available?

Ace the Data Science Interview